1. Field of the Invention
This invention relates to cameras. In particular, it relates to a filter for an autofocus mechanism of the camera.
2. Background of the Invention
There has been much recent interest in so-called compact cameras using 35 mm film. These cameras are smaller than a single lens reflex camera and lack many of the sophisticated features of the latter. With their decreased size, compact cameras can be easily carried about and their simple usage appeals to many inexperienced photographers.
However, some compact cameras have been designed with more sophisticated features. An example is an autofocusing zoom-lens compact camera which is described in PCT Patent Application PCT/JP No. 87/00293 filed at the Japanese Patent Office on May 12, 1987 with the United States being designated for filing. This application, incorporated herein by reference, is commonly assigned with this application.
The lens position is focused in accordance with a measured distance U between film plane F and the object O. The distance measuring device or range finder disclosed in the above application is schematically represented in FIG. 1 and relies upon a triangulation method. A light emitting diode 10 emits infrared radiation which is collimated by an emitting lens 12 of focal length f into a well defined beam to fall on the object O. The infrared light is reflected, usually diffusely, from the object O and part of the reflected light travels back to the camera where it is focused into a spot on a linear photoarray 14 by a receiving lens 16, also of focal length f. The separations between the light emitting diode 10 and the emitting lens 12 and between the receiving lens 16 and the photoarray 14 are both equal to the focal length f. The linear photarray 14 has many pixels and only one or two of the pixels will usually output an appreciable electrical signal when the reflected beam is focused on the photoarray. The displacement of the so activated pixel from the center of the linear photoarray is represented by t. Further, let the distance from the center of the photoarray 14 and the light emitter 10 be represented by L and the axial distance between the light emitting diode 10 and the film plane F be represented by d. Then, t is related to U by the equation EQU t=Lf/(U-f-d). (1)
The electronic processing circuit for converting the measured value of t into that for U and hence for the required displacement of the main lens is not directly related to this invention and is not described because it should be obvious to the ordinary designer.
The above design presents several problems however. A compact camera is intended to be carried around without a carrying case. For that purpose, there is provided a shutter for the main lens. However, in a low cost compact camera, it would be desirable to not have to provide a corresponding shutter for the lenses 12 and 16 of the range finding section.
Another problem is that the infrared light emitted from the range finding section is relatively weak. Although the photoarray 14 is designed to be sensitive mainly to infrared radiation, a high level of ambient visible radiation will cause spurious signals to be output from the photoarray 14.